Waste heat is a significant global environmental issue, but it is one that draws little attention because of very limited solutions. A large fraction of energy used in industrial processes and transportation systems is lost as low grade waste heat, and several trillion dollars of energy generated from fossil fuels in the U.S. each year, equal to nearly sixty quadrillion BTUs, is discarded without benefit in the form of waste heat.
Thermoelectric generators or (TEGs) using solid state conversion of heat to electricity is one technology for application to waste heat recovery, but TEGs are currently only used in niche markets because the resulting cost per kilowatt is high, and they are difficult to integrate into waste heat sources because of their flat plate form factor. TEGs are usually manufactured in small, flat plate modules using small pellets of N-type and P-type crystalline semiconductor materials wired in series or parallel. As recognized by the present inventor, the flat form factor makes the TEGs difficult to thermally couple to complex shaped heat sources and equally difficult to couple to active cooling sources, both issues adding significantly to TEG installation complexity and cost. The present inventor also recognized that TEGs made using semi-conductor pellets also require the use of solders to hold the multilayer thermoelectric couple element assembly together, and these materials often limit potential applications and the maximum electrical output of the devices. The overall design of current commercial thermoelectric devices also make them subject to degradation or failure when subjected to moderate to intense mechanical and thermal shock environments.
Additive manufacturing of TEGs offers the potential to spray or otherwise deposit the N-type and P-type semiconductor materials and all the other material layers required for a functional TEG directly onto complex shaped waste heat or other thermal energy sources. The potential combination of several additive manufacturing processes enable sequentially building up the electrical isolation layer, an adhesion layer, the interconnecting conductive metallic layer or layers, diffusion barrier material layer, and both the N-type and P-type semiconductor layers required for a functional TEG. One potential method of achieving both the deposition of the thermoelectric semiconductor and metallic materials is the use of the supersonic cold-spray deposition process, although the industry has struggled to find a solution that achieves that objective.
“Supersonic cold spray” is a material deposition process that has been developed to build up metallic material layers by impacting micrometer sized metal particles at high velocities onto a substrate. A helium or nitrogen gas stream under pressure is accelerated to supersonic velocity by expansion through a converging-diverging nozzle. The normally spherical metal particles of the material being deposited are inserted into the gas stream either in the converging or diverging sections of the nozzle and then accelerated to high velocity. The normally spherical metal particles in the size range from 10-80 micrometers become entrained within the gas and are directed towards the surface where they deform and knit together on impact forming a strong bond with the surface and with each other. Gas type, gas pressure, gas temperature, nozzle configuration, nozzle extension length, average particle size of the material being sprayed, the particle's drag coefficient, and the particle size distribution must be optimized for each different material. In addition, the feed mechanism and the feed rate of the powdered material into the gas stream must be tailored to the material being sprayed. A unique advantage of the cold-spray process is that the particles are maintained below their melt temperature, and successful deposition depends on the micrometer sized, normally spherical, metal particles deforming on impact. Thus, implementation of the cold spray process has been primarily focused on the use of metallic materials, materials that are malleable and that can be hammered or pressed permanently out of shape without breaking or cracking and particles that can be fused or forged together below their melt temperature. For that reason, the cold-spray process has not been generally applied to the deposition of crystalline or polycrystalline materials, and thermoelectric semiconductors materials and other energy harvesting semiconductor materials. In addition, in the deposition of metals, the metal particle sizes are generally restricted to being greater than ten microns in diameter and normally in the range between 25 microns and 75 microns in diameter since they must exhibit sufficient drag area and mass to be accelerated by the gas stream and gain sufficient momentum to hit the surface with enough force to deform and adhere to the surface and each other before being swept away by the gas stream.